1. Field of the Invention
The present invention relates to an exposure control apparatus for a camera and more particularly is directed to an improvement in such exposure control apparatus provided with multi metering means in which a plural number of divisional sections of an object field are metered photometrically to obtain a plural number of photoelectric outputs corresponding to the respective sections and a proper photometer output is operated and extracted from the photoelectric outputs to determine a proper exposure value for the whole object field.
2. Description of the Prior Art
The above mentioned type of multi metering device is well known in the art and there have been proposed various multi photometers for this purpose one example of which is disclosed in Japanese Patent Application laid open No. 12,828/1977 (its corresponding German Patent Laid Open Specification is DOS P No. 2,632,893).
However, all of the improvements hitherto proposed for such type of divisional photometer have been directed solely to an operational method and apparatus for calculating a proper exposure value from a plural number of informations obtained by photometering.
In practical use of known multi photometers in a camera, there arises such problem that the photoelectric outputs from the multi photometer can not be used directly as a proper photometer output because of the difference between the distribution of the photoelectric outputs in the object field and the distribution of illumination on the film plane. Hereinafter, the object field is referred to as the picture plane.
In TTL open aperture metering, the above problem becomes particularly important. In this case, the difference between the distribution of photoelectric outputs obtained at the time of TTL photometering with fully open aperture and the distribution of illumination on the film plane obtained at the time of photographing with an aperture value then actually used becomes remarkably large and therefore the difference can not be ignored at all. This problem occurs even when the light receiving plane for multi metering is disposed conjugated with the film plane. Since a light receiving optical system is provided for the former plane, the above mentioned undesirable phenomenon can not be avoided even in such case.
In the conventional photometering apparatus operable with one photoelectric output, the central part of a picture plane has been used as a main area to be metered. Since the central area of a picture plane generally exhibits good proportionality to lens aperture for both of the photoelectric output from the photo receptor and the illumination on the film plane, the above mentioned difference in distribution has no remarkable effect on the determination of proper exposure value. Therefore, in this single type of photometer it is seldom that such difference in distribution leads to exposure error. In contrast, in the case of multi photometer, the peripheral part of a picture plane is also to be metered independently of the main part and therefore the difference in distribution mentioned above can not be ignored.
As an example, it is assumed that a picture plane is divided into segments in the form of 4.times.6 matrix as shown in FIG. 1 and these 4.times.6 segments are to be metered by photo receptors P11 to P46 respectively. If the object is a surface having a uniform brightness all over, then the distribution of photoelectric output along A-A' in the picture plane shown in FIG. 1 will give a curve as shown in FIG. 2A. As seen from FIG. 2A, the level of photoelectric output drops down gradually from the center of the picture plane to both side end portions due to the vignetting effect of the lens and the effect of so-called Cos.sup.4 law (in FIGS. 2A and 2B, the distribution is plotted with the center of the picture plane as 0 and the locations of metered segments within the picture plane as the abscissa). However, when the aperture is stopped down for actually taking a picture, the effect of vignetting disappears and the distribution of illumination on the film plane gives a flattened curve (FIG. 2B) as compared with the curve of FIG. 2A. To obtain a proper exposure value, this difference between the distribution curves FIG. 2A and FIG. 2B has to be taken into consideration. Otherwise, the metered brightness for the peripheral sections of a picture plane will be unduly darker value than the real brightness thereof. This may lead the exposure to error. All the multi photometered hitherto proposed have no means for solving the problem.